1. Field of the Invention
The present invention is related to the area of display devices. More particularly, the present invention is related to techniques of using a liquid crystal display without subpixels or color filters to display colors from colored backlighting.
2. Description of the Related Art
Liquid crystal display (LCD) is one of the popular display devices for conveying digital information, such as images, data and movies. The decreased weight and size of a flat panel display greatly increases its versatility over a cathode ray tube (CRT) display. High quality flat panel displays are typically back-lit. That is, a source of illumination is placed behind the LCD layers to facilitate visualization of the resultant image. Flat panel LCD devices or units are used today in many applications including the computer industry where flat panel LCD units are an excellent display choice for lap-top computers and other portable electronic devices. Because the technology of flat panel LCD units is improving, they are being used more and more in other mainstream applications, such as desktop computers, high-end graphics computers, and as television and other multi-media monitors.
Most computer LCD panels are back-lit with built-in fluorescent tubes behind the LCD. A white diffusion panel behind the LCD redirects and scatters the light evenly to ensure a uniform display. This is known as a backlight or backlighting. One of the commonly used fluorescent tubes in LCD panels is a tiny Cold Cathode Fluorescent Lamp (CCFL) for the backlight. It provides a bright white light source that can be diffused by the panel behind the LCD. In addition to providing ample lighting, CCFLs do not rise far above the ambient temperature. This makes them ideal for LCD panels since the light source is in close proximity to other components that could be ruined by excessive heat.
One amazing thing about these CCFLs is their incredible size. They are very thin and the board that drives a lamp is very small as well. However, it is not that hard to break them, which is why the LCD display may go dark if a laptop computer drops onto a hard floor but still works.
While everyone appears to be focused on the “zero radiation” advantage of the LCD technology, no one actually think what is behind the liquid crystals (which the “L” and “C” letters stand for). This happened to be the same fluorescent light technology which was not recommended for use as the only light source in offices.
A fluorescent light is most often a long straight glass tube that produces white light. Inside the glass tube there is a low-pressure mercury vapor. When ionized, mercury vapor emits ultraviolet light. Human eyes are not sensitive to ultraviolet light. The inside of a fluorescent light is coated with phosphor. Phosphor is a substance that can accept energy in one form and emit the energy in the form of visible light. For example, energy from a high-speed electron in a commonly seen TV tube, also referred to as cathode ray tube or CRT, is absorbed by the phosphors that make up the pixels. The light from a fluorescent tube is the light given off by the phosphor coating the inside of the tube. The phosphor fluoresces when energized, hence the name. It has been concluded that directly staring at a source of fluorescent light can be just as bad, if not worse. But there is no mentioning anywhere that that most LCD panels are actually a reflection of fluorescent lights and in many cases people spend a considerable amount of time in a day staring at such a source.
Another one of the reasons that a fluorescent light is used as a backlighting source is the inherent characteristics of being close to the sunlight. However, the spectrum of a typical fluorescent light is no close to that of the sunlight, often requiring compensation on white balance. No or improper adjustments on white balance would result in distortions of colors.
U.S. Pat. No. 6,657,607 proposes a solution of using multiple light sources for color balancing within a liquid crystal flat panel display unit. In particular, altering the brightness of two or more light sources, having differing color temperatures, is thus providing color balancing of a liquid crystal display unit within a given color temperature range. The patent, however, corrects only one aspect of the problems in a liquid crystal display that is backlit by fluorescent lighting by introducing additional lighting sources and polarization means.
Light emitting diodes provide an alternative light source to the fluorescent light source. Besides having a much longer operating life, the light emitting diodes provide steady and pure colors. The current applications of the light emitting diodes in LCD are largely to combine the colored lights from the light emitting diodes for a simulated white light. The inventors herein have conceived that the light emitting diodes as the backlighting, if designed properly, can be used to enhance many properties of LCD.